1. Field of the Invention
This invention relates to joint assemblies for ducting and, more particularly, to pivot pins interfitting into gimbal joints used to flexibly connect sections of high pressure aircraft ducting together so that the sections of ducting can swivel at the joints relative to each other in a manner permitted by the invention that greatly reduces the degree of frictional stress in the joint.
2. Background of the Invention
In the aerospace industry, high pressure ducting must be flexible to accommodate the irregular internal passages of the various aircraft in which the ducting is used. To meet this requirement, sections of ducting are connected together using conventional gimbal joints. Gimbal joints provide the necessary flexibility during installation of the ducting in the aircraft. Such joints also allow the ducting limited freedom of movement when the ducting is subject to environmental stresses when the aircraft is in service.
For example, high temperatures within the ducting caused by the aircraft's hot exhaust passing through the ducting can be a substantial source of environmental stress on the ducting and joints. Fluctuations in pressure can also cause stress on the ducting and joints. To respond to such environmental pressures, the joints in the ducting are designed to allow angular fluctuations. In this way, some of the stress is released in the form of rotational movement about the axis of the joints.
The problem with this approach is that the frictional forces within the joint, which are located between the surface of the pivot pin and the walls of the joint, resist the rotational movement. These frictional forces must be overcome as joint rotation occurs. The environmental stresses are sufficient to overcome the frictional forces within the joint, however, the joint suffers as a result. In fact, the repeated frictional strain in the joint has a cumulative effect. After prolonged use, the pivot pin becomes fatigued and breaks. When breakage of the pivot pin occurs, the environmental stresses that would normally be harmlessly released in the form of rotation energy about the joint build up within the ducting causing possible fatigue and failure of other components of the ducting system.
To overcome this problem of pivot joint failure in the prior art, the present invention reduces the level of harmful friction within the joint by providing a means for continuously lubricating the joint. The pivot pin of the present invention has two structural features that accomplish this goal. The first is a reservoir located at the base of the head of the pivot pin. The reservoir is filled with lubricant when it is installed. As there is rotation about the pin pivots, small quantities of lubricant are released and leak into the joint. The rate of leakage of lubricant into the joint is expected to meet the needs of the joint over the life of the joint.
The second unique structural feature of the pivot pin of the present invention relates to the surface of a rod portion of the pivot pin. The surface of the rod portion is textured. By providing a textured surface, the lubricant released from the reservoir is captured within the depressions of the textured surface of the rod. In this way, the lubricant is trapped between the surface of the rod and the interior walls of the bore of the joint into which the rod of the pivot pin fits. Thus, the lubricant is localized where it will do the most good to reduce friction within the joint. By the structural combination of a lubricant reservoir and textured surface, the frequency of pivot pin failure caused by friction within the joint can be substantially reduced.